Many industrial processes require precise flow sensing and control of various process fluids. For example, in the pharmaceutical and semiconductor industries, mass flow sensors and mass flow controllers are used to precisely measure and control the amount of a process fluid that is introduced to a process chamber. The term fluid is used herein to describe any type of matter in any state that is capable of flow. It is to be understood that the term fluid applies to liquids, gases, and slurries comprising any combination of matter or substance to which controlled flow may be of interest.
Often, a mass flow meter is included as an integral component of a mass flow controller. In a conventional mass flow controller, the mass flow rate of a fluid flowing in a main fluid flow path is regulated or controlled based upon a mass flow rate of a portion of the fluid that is diverted into a typically smaller conduit forming a part of the mass flow sensor. Assuming stable flow in both the main flow path and the conduit of the sensor, the mass flow rate of the fluid flowing in the main flow path can be determined (and regulated or controlled) based upon the mass flow rate of the fluid flowing through the conduit of the sensor.
FIG. 1 illustrates a conventional mass flow controller 10 which includes a mass flow meter 14 having a mass flow sensor 62 and a pressure dropping bypass 42, a control valve 92, a valve actuator 94, and control electronics 46 (e.g., controller). In operation, the flow meter measures the mass flow rate of a fluid in a flow path and provides a signal indicative of that flow rate. Typically, the measurement of the flow rate is achieved with the flow sensor measuring a portion of the overall flow through the flow meter (e.g., the flow through a flow sensor conduit 68). A control valve is positioned in the fluid flow path (typically downstream of the bypass and mass flow sensor) and can be controlled (e.g., opened or closed) to vary the mass flow rate of fluid flowing through the fluid flow path and provided by the mass flow controller. The valve is typically controlled by a valve actuator, examples of which include solenoid actuators, piezoelectric actuators, stepper actuators, etc. Control electronics control the position of the control valve based upon a set point indicative of the mass flow rate of fluid that is desired to be provided by the mass flow controller, and a flow signal from the mass flow meter indicative of the actual mass flow rate of the fluid flowing in the sensor conduit 68 of the sensor 62. Traditional feedback control methods such as proportional control, integral control, proportional-integral (PI) control, derivative control, proportional-derivative (PD) control, integral-derivative (ID) control, and proportional-integral-derivative (PID) control are then used to control the flow of fluid in the mass flow controller. In each of the aforementioned feedback control methods, a control signal (e.g., a control valve drive signal) is generated based upon an error signal that is the difference between a set point signal indicative of the desired mass flow rate of the fluid and a feedback signal that is related to the actual mass flow rate sensed by the mass flow meter.
The mass flow sensor 62 measures the mass flow rate of fluid in the sensor conduit 68 that is fluidly coupled to the pressure dropping bypass 42 disposed in the body 16 of the flow meter 14. The mass flow rate of fluid in the sensor conduit is approximately proportional to the mass flow rate of fluid flowing through the bypass, with the sum of the two being the total flow rate through the flow path controlled by the mass flow controller and the ratio of fluid flow through the sensor conduit to fluid flow through the bypass remaining substantially constant.
A thermal mass flow sensor is a type of flow sensor that is commonly employed in a mass flow controller. A thermal mass flow sensor includes a thermal sensing element, for example, a pair of resistive windings (e.g., resistors) 76, 78 that are wound about the sensor conduit 68 at spaced apart positions, each having a resistance that varies with temperature. In general, an electrical current is provided to the resistive windings. The heat generated by the resistive windings is used to heat the fluid flowing through the sensor conduit 68 to a temperature that is greater than the temperature of the fluid flowing through the bypass 42. As is known to those of skill in the art, the preceding approach allows the rate of fluid flowing in the flow sensor 62 to be determined using any one of a number of different methods. For example, a constant current mass flow sensor employs a constant current to each of the upstream and downstream resistors and compares a difference in voltage across the resistors to determine the mass rate of flow of the fluid through the sensor conduit. A constant temperature mass flow sensor maintains the upstream and downstream resistors at the same predetermined value of resistance (and thus, temperature) independently of the rate of fluid flow through the sensor conduit. The difference in energy required to maintain each of the resistors at the predetermined temperature is measured and is proportional to the mass flow rate of fluid flowing through the sensor conduit.
Historically, the accuracy of flow sensors was determined relative to the full scale range of the flow sensor. For example, a flow sensor with a full scale range of 30 sccm has a +1% error where the output of the flow sensor, based on the sensed flow through the flow sensor, is within +0.3 sccm of the actual flow. This traditional approach results in greater accuracy at higher flow rates (relative to the full scale range) with decreasing accuracy at the lower end of the flow range of the flow sensor. That is, in the preceding example, 0.3 sccm is only 1% of the full scale flow but is 10% of a 3 sccm flow. The decrease in accuracy at the low end of the flow sensor range limits the range in which a particular flow sensor can be effectively employed. Further, the preceding illustrates that, where accuracy is determined relative to full scale, a high accuracy at low flows may require a much higher accuracy at high flow rates.
More recently, industries such as semiconductor manufacturing have shifted to a standard in which accuracy of flow sensors is determined relative to the flow set-point. This approach requires greater absolute accuracy at the low end of flow sensor range when compared to the accuracy at the high end of the flow sensor range. For example, where the required accuracy is +1% of the set-point, the required accuracy is +0.03 sccm when the set-point is 3 sccm and +0.3 sccm when the set-point is 30 sccm.
To meet present standards for accuracy, thermal mass flow sensors are typically restricted to a relatively small usable range. For example, the usable range of a flow sensor includes the range of sensed flow in which the sensor can meet the accuracy requirements of the application. The two examples above demonstrate the challenges faced by flow sensor designers to maintain flow sensor accuracy to within specifications regardless of whether accuracy is determined as a percentage of the full scale range of the flow sensor or as a percentage of set-point.
In a conventional flow meter, the split in flow between the flow sensor conduit and the bypass varies approximately linearly with a change in flow. Although it is generally known that a non-linear split-ratio of the flow through the flow sensor conduit and the bypass can be achieved through the use of a single orifice, conventional approaches seek to linearize the output. For example, U.S. Pat. No. 3,559,482, to Baker et al., issued Feb. 2, 1971 and entitled “Fluid Flow Measuring Apparatus,” (hereinafter “the '482 patent”) describes that a fluid flow to a sensing portion of a flow sensor can be reduced by splitting the flow into three parallel flow paths so that the flow meter can be used to measure a mass flow of much greater magnitude. The '482 patent also describes the use of an orifice plate on an inlet side of a flow sensor flow path (shunt path) in combination with a laminar flow element and a resulting flow in a shunt path that varies as a square root of the flow through the laminar flow element. The '482 patent, however, describes that a square-law meter is employed to provide a net linear output versus flow.
Other structural elements have been included in flow sensors, and in particular in the flow sensor conduit. For example, U.S. Pat. No. 5,763,774, to Ha et al., issued Jun. 9, 1998, and entitled “Fluid Flow Meter with Reduced Orientation Sensitivity,” (hereinafter the '774 patent) describes a mass flow sensor including a sensing conduit with a wire disposed therein to reduce the internal diameter of the sensing tube, and consequently, the thermal siphoning through the sensor. The approach described in the '774 patent, however, provides a linear relationship between the pressure drop across the sensor and the flow rate (i.e., the volumetric flow rate) of the sensor flow path.